Do you have Flight Modes enabled? Just asking because if they are active, it limits the number of other switches that can be used to control program mixers. As far as I know, you will not be able to use any of the D/R switches to control a mixer, not withstanding the D/R switch that is used to select Flight Modes if they are enabled.

I don't have a TX here with me so I can't be sure, but if Flight Modes are not active, you might be able to use the 3-position Land/Flap switch to control 2 different mixers to get the job done.

Len